Conventionally, each pixel circuit, arranged in an organic light emitting diode (OLED) display device and for controlling the brightness of an organic light emitting diode, is implemented with two transistors and one capacitor. However, most of the existing display panels have the non-uniformity issue due to the circuit design flaw in the conventional pixel circuit. The detail will be described in the following with a reference of FIG. 1.
FIG. 1 is a schematic view of a conventional pixel circuit. As shown, the conventional pixel circuit 100 includes a transistor 101, a transistor 102, a capacitor 103 and an organic light emitting diode 104. The transistor 101 is configured to have its gate for receiving a scan signal SCAN and its first source/drain for receiving display data DATA. The transistor 102 is configured to have its gate electrically coupled to the second source/drain of the transistor 101 and electrically coupled to a power supply voltage OVDD and its first source/drain through the capacitor 103. The organic light emitting diodes 104 is configured to have its anode electrically coupled to the second source/drain of the transistor 102 and its cathode electrically coupled to a power supply voltage OVSS; wherein the power supply voltage OVSS is smaller than the power supply voltage OVDD. According to the aforementioned circuit structure, it is noted that the current flowing through the organic light emitting diodes 104 is controlled by the crossing voltage between the gate and the second source/drain of the transistor 102. That is, IOLED=K*(VGS−|Vth|)2; wherein IOLED is referred to the current flowing through the organic light emitting diode 104, K is a constant, VGS is referred to the crossing voltage between the gate and the second source/drain of the transistor 102 and is related to the voltage of the display data DATA, and Vth is referred to the threshold voltage of the transistor 102.
However, because the metal wires, for connecting each pixel circuit 100 to the power supply voltage OVDD, may have impedances, the IR-drop may occur when the power supply voltage OVDD is driving the organic light emitting diodes 104 to illuminate light. Thus, the pixel circuits 100 may have different pixel currents IOLED and consequentially the organic light emitting diodes 104 may have different brightness. As a result, the non-uniformity issue occurs. In addition, the transistor 102 in each pixel circuit 100 may not have the same threshold voltage Vth due to the impact of the manufacturing process. Similarly, the pixel circuits 100 may have different pixel currents IOLED and consequentially the organic light emitting diodes 104 may have different brightness. As a result, the non-uniformity issue also occurs.